One of the early iPhone X adopters few months ago mentioned about the same issue on Reddit and added that while Pano, slow-mo, and time lapse modes worked fine with rear camera, everything else didn't. While these features helped the company to attract customers, it also made it hard for the service centers to perform repairs on the device. Now, it may sound weird that fixing the rear camera addressed Face ID issues, there have been known cases where users who had issues with their rear cameras also turned up having issues with Face ID. Service providers will run audio diagnostics and if the message "device could not detect dock" or "accessory not supported" pops up then further work is required. If the test identifies a problem, specialists are then asked to perform the necessary fix and see if it addresses the Face ID issue.
In a leaked internal document obtained by MacRumors, Apple warns Authorized Service Providers that iOS 11.3 and iOS 11.3.1 have rendered some phones unable to use their microphones during calls or be heard during FaceTime video chats. If the fix does not bring results, the problem persists, it is recommended to have a complete replacement iPhone X new good smartphone.
"I was talking to the Genius Bar guy and he said that the cameras are connected".
While we still don't know how the rear and front camera might be linked, it seems that a problem with the dual-lens camera found on the back of the iPhone could somehow create a problem for Face ID. The Face ID replaced the biometric scanner which was first introduced with iPhone 5s.
Basically, the document states that the employees first have to run the diagnostics test to see it the customer's Face ID problem can be solved by repairing the rear camera or not. "The telephoto is connected to the true depth and was not working", the Reddit user explained.